R&D Report 1974-19 : Digital sound: Feasibility of changing frequency pre-emphasis by a digital filter
This report investigates the feasibility of using a digital filter to change the pre-emphasis applied to an audio signal encoded in digital form from the CCIR (50 microseconds) characteristic to the CCITT characteristic. This filter is required at the interface between bit-streams carrying 13-bit and companded 10-bit digital sound signals. The disadvantage of using an analogue method for this purpose is that a separate filter would be needed for each audio channel in the multiplexed signal, preceded by a digital-to-analogue converter and followed by an analogue-to-digital converter.
It is shown that satisfactory results would be obtained by using a relatively simple (second-order) digital filter, and with filter coefficients specified to 8-bit accuracy. Some hardware aspects are considered in the report, and it is concluded that, by using suitable time sharing techniques and appropriate data storage, one central (multiplier) unit could serve a group of six multiplexed audio channels, which would be convenient in one application of the companded digital sound system.